The invention features methods, compositions, and kits for selective inhibition of pain- and itch sensing neurons (nociceptors and pruriceptors) by drug molecules of small molecule weight, while minimizing effects on non-pain-sensing neurons or other types of cells. According to the method of the invention, small, hydrophilic drug molecules gain access to the intracellular compartment of pain-sensing neurons via entry through receptors that are present in pain- and itch-sensing neurons but to a lesser extent or not at all in other types of neurons or in other types of tissue.
Local anesthetics such as lidocaine and articaine act by inhibiting voltage-dependent sodium channels in neurons. These anesthetics block sodium channels and thereby the excitability of all neurons, not just pain-sensing neurons (nociceptors). Thus, while the goal of topical or regional anesthesia is to block transmission of signals in nociceptors to prevent pain, administration of local anesthetics also produces unwanted or deletrious effects such as general numbness from block of low threshold pressure and touch receptors, motor deficits from block of motor axons and other complications from block of autonomic fibers. Local anesthetics are relatively hydrophobic molecules that gain access to their blocking site on the sodium channel by diffusing into or through the cell membrane. Permanently-charged derivatives of these compounds (such as QX-314, a quaternary nitrogen derivative of lidocaine), which are not membrane-permeant, have no effect on neuronal sodium channels when applied to the external surface of the nerve membrane but can block sodium channels if somehow introduced inside the cell, for example by a micropipette used for whole-cell electrophysiological recording from isolated neurons. Pain-sensing neurons differ from other types of neurons in expressing (in most cases) the TRPV1 receptor/channel, activated by painful heat or by capsaicin, the pungent ingredient in chili pepper. Other types of receptors selectively expressed in various types of pain-sensing and itch-sensing (pruriceptor) neurons include but are not limited to TRPA1, TRPM8, and P2X(2/3) receptors.
Neuropathic, inflammatory, and nociceptive pain differ in their etiology, pathophysiology, diagnosis, and treatment. Nociceptive pain occurs in response to the activation of a specific subset of peripheral sensory neurons, the nociceptors by intense or noxious stimuli. It is generally acute, self-limiting and serves a protective biological function by acting as a warning of potential or on-going tissue damage. It is typically well-localized. Examples of nociceptive pain include but are not limited to traumatic or surgical pain, labor pain, sprains, bone fractures, burns, bumps, bruises, injections, dental procedures, skin biopsies, and obstructions.
Inflammatory pain is pain that occurs in the presence of tissue damage or inflammation including postoperative, post-traumatic pain, arthritic (rheumatoid or osteoarthritis) pain and pain associated with damage to joints, muscle, and tendons as in axial low back pain.
Neuropathic pain is a common type of chronic, non-malignant pain, which is the result of an injury or malfunction in the peripheral or central nervous system and serves no protective biological function. It is estimated to affect more than 1.6 million people in the U.S. population. Neuropathic pain has many different etiologies, and may occur, for example, due to trauma, surgery, herniation of an intervertebral disk, spinal cord injury, diabetes, infection with herpes zoster (shingles), HIV/AIDS, late-stage cancer, amputation (including mastectomy), carpal tunnel syndrome, chronic alcohol use, exposure to radiation, and as an unintended side-effect of neurotoxic treatment agents, such as certain anti-HIV and chemotherapeutic drugs.
In contrast to nociceptive pain, neuropathic pain is frequently described as “burning,” “electric,” “tingling,” or “shooting” in nature. It is often characterized by chronic allodynia (defined as pain resulting from a stimulus that does not ordinarily elicit a painful response, such as light touch) and hyperalgesia (defined as an increased sensitivity to a normally painful stimulus), and may persist for months or years beyond the apparent healing of any damaged tissues.
Pain may occur in patients with cancer, which may be due to multiple causes; inflammation, compression, invasion, metastatic spread into bone or other tissues.
There are some conditions where pain occurs in the absence of a noxious stimulus, tissue damage or a lesion to the nervous system, called dysfunctional pain and these include but are not limited to fibromyalgia, tension type headache, irritable bowel disorders and erythermalgia.
Migraine is a headache associated with the activation of sensory fibers innervating the meninges of the brain.
Itch (pruritus) is a dermatological condition that may be localized and generalized and can be associated with skin lesions (rash, atopic eczema, wheals). Itch accompanies many conditions including but not limited to stress, anxiety, UV radiation from the sun, metabolic and endocrine disorders (e.g., liver or kidney disease, hyperthyroidism), cancers (e.g., lymphoma), reactions to drugs or food, parasitic and fungal infections, allergic reactions, diseases of the blood (e.g., polycythemia vera), and dermatological conditions. Itch is mediated by a subset of small diameter primary sensory neurons, the pruriceptor, that share many features of nociceptor neurons, including but not limited to expression of TRPV1 channels. Certain itch mediators—such as eicosanoids, histamine, bradykinin, ATP, and various neurotrophins have endovanilloid functions. Topical capsaicin suppresses histamine-induced itch. Pruriceptors like nociceptors are therefore a suitable target for this method of delivering ion channels blockers.
Despite the development of a variety of therapies for pain and itch, there is a need for additional agents.